1. Field of the Invention
The present invention is in the area of concrete reinforcements, that is, rods of metal or other materials, used to strengthen the concrete used in buildings and other construction. More specifically, the present invention encompasses a corrosion-resistant joint for metal reinforcements.
2. Description of Related Art
One of the most ubiquitous construction materials of our civilization is concrete. Everything from high rise buildings to bridges is constructed of concrete. Unhardened concrete is typically an aqueous mixture of Portland cement powder with sand and gravel. Portland cement is an extremely alkaline material produced from limestone and clay. After the liquid slurry is placed in forms, an exothermic chemical reaction occurs in which insoluble silicate crystals grow and interlock the sand and gravel grains. The forms are removed after the concrete hardens, or "cures." While the interlocking silicate structure is extremely hard, it has the drawback of being quite brittle. Therefore, if the hardened concrete is exposed to excess force, it will crumble. To overcome this problem, concrete is usually reinforced with metal or other materials to impart increased flexural strength characteristics.
The use of metal rods or "rebar" is a common and economical way to reinforce concrete. These rods, usually of steel, are available in a number of different sizes and lengths. The rods are typically cut and bent to fit the concrete structure, and the concrete is then placed around and over the rods. The reinforcing rods are completely embedded in the concrete matrix. As the mixture hardens, the concrete bonds strongly to the surface of the rods which, in turn, impart flexural strength to the concrete mass. Of course, rods do not come in the exact lengths for all concrete forms. Typically, the rods must be cut and spliced to fit a particular job. Because the final strength of the cured concrete depends on the strength of the reinforcing rods, splices must not weaken the rod. Rods are often spliced by some sort of welding process, although mechanical fasteners such as shown in U.S. Pat. No. 4,666,326 to Hope, and U.S. Pat. Nos. 3,540,763, 3,552,787, and 4,692,052 to Yee are also commonly used.
As already mentioned, the strength of the hardened concrete depends, to a great extent, on the strength of the reinforcing rods. Therefore, corrosion of the rods becomes a serious problem. Hardened concrete is somewhat porous so that air and moisture can penetrate and contact the reinforcing rods and promote oxidation (rust). Furthermore, the wet concrete itself is alkaline, which can further promote the corrosion of the metal. When rods rust, they not only lose their strength, but they also swell, causing the concrete to split. To overcome the corrosion problem, reinforcing rods are often coated with zinc (galvanized) or with plastic such as epoxy.
A difficulty with coated rods is that it is relatively easy to accidentally scrape away the coating on the rods during transport or installation. In addition, the coating must be removed prior to welding rods together.
Messy and toxic epoxy mixtures are available to coat welds and scrapes to avoid future corrosion. However, such materials are laborious to apply and often require significant time to harden properly after application. Furthermore, if the surface to be coated is not properly cleaned, or is wet, or if the ambient temperature is not correct, the coating fails to adhere properly and corrosion is not adequately inhibited.